Motor Control Center 1

[ControlsEngineer51]

I am new to MCC's. I have an existing MCC that has a 575V 1000A bus and most of my equipment is 480V. Do I need to use transformers for each bucket to step down to 480 or is there a better way to do this?

 

[resqcapt19]

You probably should look at a large step down transformer to feed a new 480 volt MCC. There would never be room in a typical MCC bucket to install a power step down transformer.

 

[wroggent]

Dumb question for clarity: is the MCC just rated at 575V or is it operating at 575V, i.e. what is the voltage of the transformer it is fed from?

 

[waross]

We used to service a plant with both 480 Volt and 600 Volt motors.
The switch gear line-up was as follows:
1> An incoming section with metering and main breaker at 600 Volts.
2> Several 600 Volt MCC sections.
3> A section with a main switch and two auto-transformers in open delta configuration dropping the voltage to 480 Volts.
4> Several 480 Volt MCC sections.
Notes:
a> Auto-transformers do not provide galvanic isolation, so the section past the auto-transformers is NOT a separately derived system and no special system grounding is required.
b> The available fault current and the ground fault current on two phases will be limited by the auto-transformers. Hopefully some-one with more arc flash experience than myself will assist with arc flash study methods.
c> With an auto-transformer feed, line to neutral or 277 volt loads are not allowed.
To allow 277 Volt loads you will need to use three auto-transformers connected in wye. Do NOT use a three phase auto-transformer with a three legged core. The phantom delta effect may present problems in the event of incoming voltage unbalances or phase angle errors.
d> Prices, quotes and purchase; Consider that from a technical viewpoint, the additional equipment, whether stand-alone or built with MCC sections need not match the existing equipment. Ask for competitive quotes from several manufacturers. Additions to existing equipment are priced as replacement parts. The replacement parts are priced about 400& higher than competitive bid prices.
Anecdote alert!
We once needed to add one section to an existing MCC. The price came in at over $20,000. The small project was about to be scrapped. I suggested that we did not have to use the same manufacturers equipment. We could bolt up any MCC section. The purchasing department went back and asked for competitive bids from 4 or 5 manufacturers. The $20,000+ price dropped to $5,000+ and some prices were $4,000+
We were able to purchase the equipment that we desired and complete the project.
Of course, if this is a 575 rated MCC running at 480 Volts, these points are moot.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ControlsEngineer51]

I should have clarified the MCC is operating at 575 off of a 600V 2000A bus. Im concerned that I will need to step down every single feed to the different equipment.

 

[waross]

I'm not doubting you, just wondering. Is this a very old installation? While 575 Volts is a standard motor rating to allow for line voltage drop, 575 equipment is generally fed from 600 Volt transformers in the areas where I work and have worked. But I haven't seen everything.
To answer your question again, step down once for the MCC sections feeding 480 Volt equipment.
Cheapest by far is to use auto-transformers.
Note standard auto-transformers will be 120:480 Volt. That will step 600 Volts down to 480 Volts.
If 120:600 auto-transformers are used to step down 575 Volts the resulting voltage will be 460 Volts.
Most motors intended for use on 480 Volt systems are actually rated for 460 Volts so it should be serviceable. If your selected transformers have voltage taps, then set the taps for maximum voltage. +5% taps should give 483 Volts. Close enough.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ControlsEngineer51]

Waross- this is a pretty old install not to mention it is in Canada.

Im going to look into auto transformers and see if this is the cheapest option. Should I be at concerned with noise if im adding 3-4 transformers?

 

[waross]

Generally the cheapest option is to add two transformers in open delta auto-transformer configuration. If you have 277 Volt loads, you must use three auto-transformers. The three transformer option takes a little more labour and space and may cost a little more. 480:120 Volt transformers for the two transformer option may be easier to source than the 277:120 Volt transformers used with the three transformer option.
The three transformer option looks good on paper. It is symmetrical and the vector diagram is easy to understand, but in practice the two transformer option is often preferred.
By the way, I have worked on plant shutdowns where the plant voltage was 600 Volts and the rental generators for temporary power were 480 Volts.
Auto-transformers were used to boost the 480 Volts up to 600 Volts for the plant equipment. Then some 480 Volt rental equipment came in. A second set of auto-transformers was used to drop the 600 Volts back down to 480 Volts. One set of auto-transformers was two transformer open delta, and the second set of auto-transformers was three transformer wye. No problems with either set.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

You are over thinking this. The MCC bus is not a consumer, it is a delivery system. All MCC bus systems are rated at 600V maximum. What it delivers is what you hook up to the line terminals. If you hook it up to 480V, it distributes 480V to the motor starters. If you connect 460V motors to the starters, they run on 460V.

The only POSSIBLE glitch is if each individual starter bucket has its own Cpntrol Power Transformer (CPT) to provide 120V control power. If it was 600V, the primary of those CPTs will be 600V so you will not be able to use them with a 480V input, you will need to change them to 480:120V CPTs. Just match the kVA rating of them.

Another more minor issue will be the overload relays. A motor of the same HP rating will take more current at 460V than it did at 575V. But you should always check the OL heaters and/or adjustments for a new installation anyway. So just get the user manuals for the OL relays to determine the correct way to select the heaters or settings, and do what it says.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

Hi Jeff. Years ago, we had a customer who brought in a lot of equipment from Ontario Canada. Almost anything out of Ontario back then was 575 Volts. We had pairs of auto-transformers all over his plant correcting the voltage. This was the plant with the dual voltage MCC, one section 600 Volts and one section 480 Volts. Then he would replace a 600 Volt machine with a 480 Volt machine. There was no easy way to change the underground conduit routing from one section of the MCC to the other section, so a small pair of auto-transformers would go in beside the machine. 600 Volts is more common in Canada than in the US. With the 600 Volt supplies go the 575 Volt rated motors.
I believe that the OP is adding 480 Volt equipment to an existing 575/600 Volt system. I may be wrong.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MatthewDB]

In response to Bill's suggestion on 3 transformer setups: 600V is 343V L-N. You need a 277:69 transformer, not 277:120.

We get ours special order.